Conventionally, various methods for producing cycloolefin have been known. Among them, a method for partially hydrogenating a monocyclic aromatic hydrocarbon using a ruthenium catalyst in liquid phase is most typical. Furthermore, many results of investigations into catalyst components, types of carriers, metal salts as additives to the reaction system, or the like have been reported as methods for increasing selectivity or yield of cycloolefin when the cycloolefin is produced.
In an apparatuses used when cycloolefin is produced by such a method, material corrosion progresses in the portion (hereinafter, referred to as liquid-contact portion) where liquid contacts a reactor or the like by anion and/or alkali as additives. As a result, it has been known that there occurs a phenomenon where activity and/or selectivity of catalysts are/is reduced by a metallic component which elutes from the liquid-contact portion, or the like. Thus, as materials of the liquid-contact portion in a reactor or the like, there are proposed means using a nickel group alloy containing molybdenum or a nickel group alloy containing chromium and molybdenum with a slow metal elution rate (Patent Document 1). Furthermore, in Patent Document 1, there are proposed a method where a reaction was performed by maintaining a concentration of nickel in an aqueous phase of a reaction liquid at 50 ppm or less. Furthermore, there is proposed a method of controlling an operation in this range where a ratio of a total amount of metals which elutes from a hydrogenation reactor and a heat exchange system to a total mass of a hydrogenation catalyst does not exceed 0.5% by mass (Patent Document 2).
As a method for regenerating a ruthenium catalyst whose activity has decreased due to an interaction between hydrogen and the ruthenium catalyst, there is proposed a method of bringing the ruthenium catalyst into contact with oxygen in a liquid phase (Patent Document 3). Furthermore, there is proposed a method of maintaining the ruthenium catalyst in a liquid phase at a hydrogen partial pressure lower than that at a hydrogenation reaction condition and at a temperature within a range from a temperature not lower than a temperature lower by 50° C. than that at the hydrogenation reaction condition to 250° C. (Patent Document 4).
There is proposed a method of restoring an activity of the ruthenium catalyst including steps of bringing the ruthenium catalyst into contact with oxygen in a liquid phase and maintaining the catalyst at a hydrogen partial pressure lower than that at a hydrogenation reaction and at a temperature not lower than a temperature lower by 50° C. than temperature of the hydrogenation reaction (Patent Document 5).    Patent Document 1: Japanese Patent No. 2925129    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-315380    Patent Document 3: Japanese Patent No. 2634828    Patent Document 4: Japanese Patent No. 2886563    Patent Document 5: International Publication No. 97/16429